(S-8) Nuclear Power

Note: This is a side-excursion into the basics of nuclear energy, beyond the main scope of "from Stargazers to Starships." It is included because nuclear energy is important to modern society, and because section S-7 "The Energy of the Sun" has already provided many of the basic ideas. Bear in mind that even without math, this can be a fairly difficult subject and that the discussion is rather lengthy.

The ideas from section S-7 are reviewed in what follows next. The rest of the section is a qualitative discussion of all key processes involved in the practical use nuclear energy.

A Review of Nuclear Structure

The way the Sun generates its energy helps understand the way a nuclear power station does so. The two processes are however quite different.

Here some facts about the way protons and neutrons combine to form nuclei, as covered in section S-7 about the Sun:

Apart from their electrical charge, protons and neutrons ("nucleons") are quite similar. They can attract other nucleons and combine with them to form heavier nuclei, a process which releases energy. For instance, on the Sun pairs of protons combine with pairs of neutrons to form helium nuclei. In the process atomic particles gain great speed, and that is how the Sun's heat is generated.

Unlike gravity or electrical forces, the nuclear force is effective only at very short distances. At greater distances, the protons repel each other because they are positively charged, and charges of the same kind repel.

For that reason, the protons forming the nuclei of ordinary hydrogen--for instance, in a balloon filled with hydrogen--do not combine to form helium (a process which also would require some to combine with electrons and become neutrons). They cannot get close enough for the nuclear force, which attracts them to each other, to become important! Only in the core of the Sun, under extreme pressure and temperature, can such a process take place.

Other small nuclei can similarly combine into bigger ones and release energy, but in combining such nuclei, the amount of energy released is much smaller. The reason is that while the process gains energy from letting the nuclear attraction do its work, it has to invest energy to force together positively charged protons, which also repel each other with their electric charge.

Once iron is reached--a nucleus with 26 protons--this process no longer gains energy (nickel also has peak binding energy). In even heavier nuclei, we find energy is lost, not gained by adding protons. Overcoming the electric repulsion (which affects all protons in the nucleus) requires more energy than what is released by the nuclear attraction (effective mainly between close neighbors).
Energy could actually by gained, however, by breaking apart nuclei heavier than iron.

In the biggest nuclei (elements heavier than lead), the electric repulsion is so strong that some of them spontaneously eject positive fragments--usually nuclei of helium, which form very stable combinations ("alpha particles"). This spontaneous break-up is one of the forms of radioactivity found in nuclei.

Nuclei heavier than uranium break up too quickly to be found in nature, although they can be produced artificially. The heavier they are, the faster is their spontaneous decay.

In summary, then: iron or nickel nuclei are the most stable ones, and the best sources of energy are therefore nuclei as far removed from iron as possible. One can combine the lightest ones--nuclei of hydrogen (protons)--to form nuclei of helium, and that is how the Sun gets its energy. Or else one can break up the heaviest ones--nuclei of uranium--into smaller fragments, and that is what nuclear power companies do.

How many Protons, how many Neutrons?

As already noted, protons and neutrons (jointly called "nucleons") are intrinsically similar, and can convert into each other, absorbing or emitting an electron to maintain proper electric charge. What determines how many of each are found in a nucleus?

The nuclear forces apparently prefer equal numbers of each kind, and light nuclei--helium, carbon, nitrogen, oxygen--usually maintain a 50:50 ratio, although nuclear variants ("isotopes") with small deviations from equality may exist and may be stable.

In heavier nuclei, because of the electric repulsion between protons, this equality no longer holds. Imagine a nucleus with 56 nucleons, and suppose we could choose how many of this total would be neutrons and how many protons. What would be the most stable combination?

Choosing 28 of each kind might provide the most stable nuclear binding, but that is offset by the energy required to hold in close quarters 28 positive protons. So nature compromises: the preferred combination--the nucleus of the most common form of iron--has 30 neutrons but only 26 protons.

As nuclei get heavier, the fraction of protons drops still further--about 45% in mid-range nuclei, and less than 40% in the heaviest ones, those of uranium. Ordinary uranium ("U-238") has 92 protons but 146 neutrons, for a total of 238 nucleons. As will be seen, this gradual change in the proton/neutron ratio is essential to the nuclear chain reaction.

As the example of France suggests, nuclear power can be the main energy source of an industrial nation, though it calls for a high level of professional competence and carefully designed safety features. At the same time, the accidents at Three Mile Island and Chernobyl are a reminder that this power source carries unique risks of its own.

Further Exploring

If you have the time and the motivation, an enormous number of sources can give you more detailed information about matters discussed here, both in print and on the web. There is, for instance, "The Virtual Nuclear Tourist", a very detailed overview of the nuclear power industry.

Twenty five years after the accident at Three Mile Island, J. Samuel Walker, a historian with the Nuclear Regulatory Commission, published "Three Mile Island: A Nuclear Crisis in Historical Perspective (315 pp, $24.94, U. Calif Press, Berkeley, 2004). Its review is found in "Science,"vol. 305, p. 181, 9 July 2004.

Another rich source of information (also covering nuclear weapons) is the book "Megawatts and Megatons: A Turning Point in the Nuclear Age?" by Richard L. Garwin and Georges Charpak, 431 pp, $30, Knopf, New York, 2001.